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United States Patent |
5,334,573
|
Schild
|
August 2, 1994
|
Sheet material for thermal transfer imaging
Abstract
Sheet materials for use in thermal transfer imaging systems comprising a
donor sheet and a receiving sheet are provided wherein the donor sheet and
the receiving sheet do not stick to each other during thermal processing.
Inventors:
|
Schild; Howard G. (Brighton, MA)
|
Assignee:
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Polaroid Corporation (Cambridge, MA)
|
Appl. No.:
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801460 |
Filed:
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December 2, 1991 |
Current U.S. Class: |
503/227; 428/419; 428/480; 428/500; 428/913; 428/914 |
Intern'l Class: |
B41M 005/035; B41M 005/38 |
Field of Search: |
8/471
428/195,913,914,419,480,500
503/227
|
References Cited
U.S. Patent Documents
4555427 | Nov., 1985 | Kawasaki et al. | 428/195.
|
4615938 | Oct., 1986 | Hotta et al. | 428/323.
|
4626256 | Dec., 1986 | Kawasaki et al. | 8/471.
|
4721703 | Jan., 1988 | Kabayashi et al. | 503/227.
|
4820687 | Apr., 1989 | Kawasaki et al. | 503/227.
|
4914078 | Apr., 1990 | Hann et al. | 503/227.
|
4997807 | Mar., 1991 | Mukoyoshi et al. | 503/227.
|
5024989 | Jun., 1991 | Chiang et al. | 503/227.
|
Foreign Patent Documents |
407615A1 | Jan., 1991 | EP | 503/227.
|
0223862 | Nov., 1985 | JP | 503/227.
|
0223878 | Nov., 1985 | JP | 503/227.
|
63-247397 | Jun., 1990 | JP | 503/227.
|
Other References
Patent Abstracts of Japan, vol. 14, No. 351.
|
Primary Examiner: Hess; B. Hamilton
Attorney, Agent or Firm: Loeschorn; Carol A., Black; Edward W.
Claims
I claim:
1. Sheet materials for use in combination in thermal diffusion transfer
imaging comprising a donor sheet and a receiving sheet, the donor sheet
comprising a support, an image-forming material capable of being
transferred by heat and a polymer system comprising at least one polymer
as a binder for the image-forming material, and the receiving sheet
comprising a polymer system comprising at least one polymer capable of
receiving said image-forming material from said donor sheet upon
application of heat thereto, the polymer system of said receiving sheet
being incompatible/immiscible with the polymer system of said donor sheet
at the receiving sheet/donor sheet interface so that there is no adhesion
between the donor sheet and the receiving sheet during thermal processing,
said donor sheet polymer system and said receiving sheet polymer system
being substantially free of a release agent.
2. The combination according to claim 1 wherein said donor sheet polymer
system and said receiving sheet polymer system are substantially free of
release agents selected from the group consisting of silicone-based oils,
poly(organosiloxanes), fluorine-based polymers, fluorine- or
phosphate-containing surfactants, fatty acid surfactants and waxes.
3. The combination according to claim 1 wherein said receiving sheet
additionally comprises a support material.
4. The combination according to claim 1 wherein said receiving sheet
polymer is an extruded polymer film.
5. The combination according to claim 1 wherein said image-forming material
is a dye.
6. The combination according to claim 1 wherein the polymer system of said
receiving sheet additionally comprises a second polymer forming a polymer
blend.
7. The combination according to claim 1 wherein said donor sheet polymer
system comprises a blend of two or more polymers as the binder for said
image-forming material.
8. The combination according to claim 1 wherein the polymer for said donor
sheet is an acrylate resin.
9. The combination according to claim 8 wherein said acrylate resin is
poly(methyl methacrylate).
10. The combination according to claim 9 wherein the polymer system for
said receiving sheet comprises poly(2,2-dimethyl-l,3-propylene succinate)
polyester.
11. The combination according to claim 10 wherein the polymer system for
said receiving sheet additionally comprises a second polyester resin
comprised of aromatic diacids and an aliphatic diol.
12. The combination according to claim 9 wherein the polymer system for
said receiving sheet comprises poly(ethylene adipate) polyester.
13. The combination according to claim 12 wherein the polymer system for
said receiving sheet additionally comprises a second polyester resin
comprised of aromatic diacids and an aliphatic diol.
14. The combination according to claim 9 wherein the polymer system for
said receiving sheet comprises poly(caprolactone) polyester.
15. The combination according to claim 14 wherein the polymer system for
said receiving sheet additionally comprises a second polyester resin
comprised of aromatic diacids and an aliphatic diol.
16. The combination according to claim 1 wherein the polymer for said donor
sheet is a poly(vinyl butyral) resin.
17. The combination according to claim 16 wherein the polymer system for
said receiving sheet comprises polystyrene
18. The combination according to claim 17 wherein the polymer system for
said receiving sheet additionally comprises a liquid crystal polymer.
19. A process for thermal diffusion transfer imaging comprising placing a
donor sheet and an image-receiving sheet adjacent to one another and
heating selected portions of the donor sheet so as to transfer said
image-forming material from the donor sheet to the receiving sheet, the
donor sheet comprising a support, an image-forming material capable of
being transferred by heat and a polymer system comprising at least one
polymer as a binder for the image-forming material, and the receiving
sheet comprising a polymer system comprising at least one polymer capable
of receiving said image-forming material from said donor sheet upon
application of heat thereto, the polymer system of said receiving sheet
being incompatible/immiscible with the polymer system of said donor sheet
at the receiving sheet/donor sheet interface so that there is no adhesion
between the donor sheet and the receiving sheet during thermal processing,
said donor sheet polymer system and said receiving sheet polymer system
being substantially free of a release agent.
20. A process for thermal imaging according to claim 19 wherein said donor
sheet polymer system and said receiving sheet polymer system are
substantially free of release agents selected from the group consisting of
silicone-based oils, poly(organosiloxanes), fluorine-based polymers,
fluorine- and phosphate-containing surfactants, fatty acid surfactants and
waxes.
21. A process for thermal imaging according to claim 19 wherein said
receiving sheet additionally comprises a support material.
22. A process for thermal imaging according to claim 19 wherein said
receiving sheet polymer is an extruded polymer film.
23. A process for thermal imaging according to claim 19 wherein said
image-forming material is a dye.
24. A process for thermal imaging according to claim 19 wherein the polymer
system for said receiving sheet additionally comprises a second polymer
forming a polymer blend.
25. A process for thermal imaging according to claim 19 wherein said donor
sheet polymer system comprises a blend of two or more polymers as the
binder for said image-forming material.
26. A process for thermal imaging according to claim 19 wherein the polymer
for said donor sheet is an acrylate resin.
27. A process for thermal imaging according to claim 26 wherein said
acrylate resin is poly(methyl methacrylate).
28. A process for thermal imaging according to claim 27 wherein the polymer
system for said receiving sheet comprises poly(caprolactone) polyester.
29. A process for thermal imaging according to claim 28 wherein the polymer
system of said receiving sheet additionally comprises a second polyester
resin comprised of aromatic diacids and an aliphatic diol.
30. A process for thermal imaging according to claim 27 wherein the polymer
system for said receiving sheet comprises poly(2,2-dimethyl-1,3-propylene
succinate) polyester.
31. A process for thermal imaging according to claim 30 wherein the polymer
system for said receiving sheet additionally comprises a second polyester
resin comprised of aromatic diacids and an aliphatic diol.
32. A process for thermal imaging according to claim 27 wherein the polymer
system for said receiving sheet comprises poly(ethylene adipate)
polyester.
33. A process for thermal imaging according to claim 32 wherein the polymer
system for said receiving sheet additionally comprises a second polyester
resin comprised of aromatic diacids and an aliphatic diol.
34. A process for thermal imaging according to claim 19 wherein the polymer
for said donor sheet is a poly(vinyl butyral) resin.
35. A process for thermal imaging according to claim 34 wherein the polymer
system for said receiving sheet comprises polystyrene.
36. A process for thermal imaging according to claim 35 wherein the polymer
system for said receiving sheet additionally comprises a liquid crystal
polymer.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a sheet material for use in a thermal transfer
imaging system comprising a receiving sheet and a donor sheet. More
particularly, it relates to a thermal imaging system wherein the donor
sheet and receiving sheet do not stick to each other during thermal
processing.
2. Description of the Related Art
Thermal transfer imaging processes wherein one or more thermally
transferable dyes are transferred from a donor sheet to a receiving sheet
in response to heat are well known. Such imaging processes employ imaging
media consisting of a donor sheet comprising a dye or dyes and a binder
for the dyes which is placed adjacent to a receiving sheet suitable for
receiving the transferred dye(s). The imaging process comprises heating
selected portions of the donor sheet in accordance with image information
to effect an imagewise transfer of the dye(s) to the receiving sheet,
thereby forming an image on the receiving sheet.
To enhance the image-receiving capability of the image-receiving sheet and
thereby obtain higher density images, resins having a low glass transition
point and softening point, e.g., polyester resins, are generally coated on
the image-receiving sheet. However, when imaging is effected, heat is
applied at high temperatures e.g., generally 200.degree. C. or higher when
a thermal printhead is employed. The high temperatures cause softening
and/or melting of the resin in the image-receiving sheet and the binder
for the dyes in the dye donor sheet resulting in adhesion between the two
sheets. This adhesion results in sticking and subsequent tearing of the
two sheets upon separation from each other.
To eliminate this thermal sticking, it has been suggested to incorporate a
dye-permeable release agent in either the donor or receiving sheet which
allows for dye transfer but prevents adhesion of the donor sheet to the
receiving sheet during printing. The release agent can be employed either
as a discrete layer on top of the receiving material or the dye layer in
the donor sheet, or the release agent can be blended in with the receiving
material before coating.
Materials previously employed as release agents include silicone-based
oils, poly(organosiloxanes), fluorine-based polymers, fluorine- or
phosphate-containing surfactants, fatty acid surfactants and waxes. The
inherently different chemical structure of the release agents from that of
the dyes to be transferred leads to an interfacial barrier at the
donor/receiver interface causing decreased dye densities in the
image-receiving sheet. These materials are surface-active which promotes
their presence at the receiving sheet/donor sheet interface where they
additionally contribute desired slip properties and frictional
characteristics to the image-receiving surface to prevent sticking.
However, these release agents tend to be migratory and can be rubbed off
the surface by touch, providing areas where sticking can occur. They also
attract dirt and dust which degrade image quality.
Crosslinking of various release materials has been proposed to hold the
release material in place and to alleviate some of the above problems.
U.S. Pat. No. 4,626,256 issued Dec. 2, 1986, U.S. Pat. No. 4,820,687
issued Apr. 11, 1989, and U.S. Pat. No. 4,914,078 issued Apr. 3, 1990
disclose image-receiving layers containing dye-permeable releasing agents
comprising hardened type (crosslinked) silicone oils. However, there are
disadvantages to having a separate crosslinked material. Not only is there
a decrease in dye density due to the inherently different chemical
structure of the silicone oils from that of the dyes, but crosslinking
additionally causes a decrease in the transferred dye density. The
temperature requirements of thermally induced crosslinking processes limit
the types of support materials that may be utilized for the receiving
sheet. Moreover, certain release materials, most notably the silicone oils
and crosslinked silicone oils, make it difficult to laminate the
image-receiving sheet to other materials because they inhibit the
laminating adhesive from adhering to the image-receiving sheet. Further,
the release materials make it difficult to write on the image-receiving
sheet because they interfere with ink adhesion at the image-receiving
surface.
It has also been suggested to increase the heat resistance of the
image-receiving material to prevent softening of the receiving material
and hence alleviate sticking. U.S. Pat. No. 4,721,703, issued Jan. 26,
1988, discloses a receiving sheet comprising a base material and a coating
composition, the coating composition consisting essentially of a
thermoplastic resin for receiving a dye and a compound having two or more
free radical polymerizable ethylenically unsaturated double bonds in one
molecule, the coating being crosslinked. The resulting receiving sheet is
described as being substantially non-heat bondable (does not stick) to the
dye layer by virtue of the heat resistance imparted by the crosslinked
polymer therein. However, this method is disadvantageous in that
crosslinked materials generally result in decreased dye densities and
require an additional processing step.
U.S. Pat. No. 4,997,807, issued Mar. 5, 1991, discloses a receiving sheet
which is described as free from blocking (sticking of the receiving sheet
to the donor sheet during thermal processing). The receiving sheet
comprises a support having thereon an image-receiving layer formed by
coating a substantially solvent-free coating composition comprising (A) a
macromonomer dyeable with a sublimable dye and containing a radical
polymerizable functional group at one terminal of the molecular chain
thereof, said macromonomer being solid at room temperature, dissolved in
(B) a liquid radiation-curable monomer and/or oligomer on a support and
irradiating the coat with radiation. According to the examples given in
the patent, excellent blocking results were obtained only when a
polyfunctional monomer and a siloxane were present. This suggests that
both crosslinking and a surface active agent (release agent) are necessary
in order to obtain the best results.
U.S. Pat. No. 4,555,427, issued Nov. 26, 1985, discloses a heat
transferable sheet (receiving sheet) comprising a receptive layer which
receives a dye transferred from a heat transfer printing sheet upon being
heated, the receptive layer comprising first and second regions having the
following properties:
(a) The first region is formed from a synthetic resin having a glass
transition temperature of from -100.degree. to 20.degree. C., preferably
from -50.degree. to 10.degree. C., and having polar groups such as an
ester linkage, C--CN linkage and C--C1 linkage.
(b) The second region is formed from a synthetic region having a glass
transition temperature of at least 40.degree. C., preferably from
50.degree. to 150.degree. C., and preferably the second region-forming
synthetic resin has also a polar group.
(c) Both the first region and the second region are exposed at the surface
of the receptive layer, and the first region occupies at least 15%,
preferably from 15 to 95% of the surface.
(d) The first region is present in the form of mutually independent
islands, the respective longitudinal length of which is from 0.5 to 200
.mu.m, preferably from 10 to 100 .mu.m, and desirably the periphery of the
first region is substantially surrounded by the second region.
According to the examples given in the patent, hardened silicone oils were
added to enhance the releasability of the heat transfer printing sheet
upon being heated.
SUMMARY OF THE INVENTION
The present invention provides a sheet material for use in thermal transfer
imaging systems which avoids sticking, i.e., the thermal fusing of the
donor sheet and the image-receiving sheet during thermal processing, by
employing an image-receiving polymer system which is
incompatible/immiscible with the donor polymer system. Since the two
polymer systems are incompatible/immiscible at the temperature and time
which they are in contact, i.e., during thermal processing, there is no
thermal adhesion between the donor sheet and the image-receiving sheet.
Specifically, the present invention provides thermal transfer imaging
systems comprising a donor sheet and a receiving sheet, the donor sheet
comprising a support, an image-forming material capable of being
transferred by heat and a polymer system comprising at least one polymer
as a binder for the image-forming material, and the receiving sheet
comprising a polymer system comprising at least one polymer capable of
receiving said image-forming material from said donor sheet upon
application of heat thereto, the polymer system of said receiving sheet
being incompatible/immiscible with the polymer system of said donor sheet
at the receiving sheet/donor sheet interface so that there is no adhesion
between the donor sheet and the receiving sheet during thermal processing,
said polymer system of the donor sheet and said polymer system of the
receiving sheet being substantially free of a release agent, such as
silicone-based oils, poly(organosiloxanes), fluorine-based polymers,
fluorine- or phosphate-containing surfactants, fatty acid surfactants and
waxes.
The present invention further provides for a method of thermal transfer
imaging employing the above described sheet materials.
By avoiding the use of a separate release agent, the present invention
provides images of higher dye densities. Since no post-coating
crosslinking is necessary, a one-step process produces the image-receiving
sheet and dye densities are not compromised. Since no heat, other than
moderate drying temperatures is required, thermal distortion of the
support material is avoided. Moreover, since the present invention lacks a
silicone oil or other low surface energy release agent, lamination of the
image-receiving sheet to other materials is easier as is writing with ink
on the surface of the image.
DETAILED DESCRIPTION OF THE INVENTION
As noted above, the sheet materials of the present invention are used in
thermal transfer imaging systems. The donor sheet comprises a support and
an image-forming material capable of being transferred by heat and at
least one polymer as a binder for the image-forming material. The
image-forming material can be a dye or other image-forming material which
transfers by diffusion or sublimation, upon application of heat, to the
image receiving sheet to form an image therein. It will be understood that
where multicolor images are desired, the donor sheet would comprise
additional dyes or other image-forming materials. The image-receiving
sheet comprises a polymer system comprising at least one polymer capable
of receiving said image-forming material from said donor upon the
application of heat thereto, the polymer system of said receiving sheet
being incompatible/immiscible with the polymer system of the donor sheet
at the receiving sheet/donor sheet interface so as to inhibit thermal
adhesion between the donor and receiving sheets during thermal processing.
The polymer system employed as binder for the image-forming material and
the polymer system of the receiving sheet are substantially free of
release agents, such as silicone-based oils, poly(organosiloxanes),
fluorine-based polymers, fluorine- or phosphate-containing surfactants,
fatty acid surfactants and waxes. "Substantially free of" means that none
of these materials are intentionally added to aid release. Selected
portions of the donor sheet are heated in accordance with image
information so as to transfer dye or other image-forming material from the
donor sheet to the receiving sheet to form an image thereon.
The image-receiving polymer system of the present invention may be coated
on a support or it may be self-supporting.
The terms incompatible and immiscible are used interchangeably but the
latter is the preferred term according to The Encyclopedia of Polymer
Science and Engineering, John Wiley & Sons, 1988, vol. 12, p. 399.
By definition, two polymers are considered to be immiscible if when they
are "in contact" (the geometry of which is very much a function of the
method of preparation, e.g., melt-mixing, solution mixing, laminating,
etc.) there is no intimate mixing, i.e., there are gross symptoms of
macroscopic phase segregation/separation into more than one phase.
In the present invention, the donor and receiving polymer systems are "in
contact" during imaging and are immiscible at the temperature and time of
contact, the latter being on the order of milliseconds, so that there is
no mixing of the two and, therefore, no thermal adhesion of the donor and
receiving sheets. Thus, while the image-receiving polymer(s) and the
binders in the donor sheet may be softened by the temperatures of thermal
processing, they are immiscible and, therefore, they do not adhere to each
other.
The donor binder serves to keep the image-forming material dispersed
uniformly and to prevent transfer or bleeding of the relatively low
molecular weight image-forming material except where the donor sheet is
heated during the thermal imaging. A necessary requirement, therefore, is
that the binder be able to dissolve and/or disperse the dye. This
necessarily excludes silicone-based oils, poly(organosiloxanes),
fluorine-based polymers, fluorine- or phosphate-containing surfactants,
fatty acid surfactants and waxes since these materials, based on their
inherent elemental structure, are not capable of keeping the dye uniformly
dispersed. Suitable binders for the image-forming material, provided they
are immiscible with the polymer system of the receiving sheet, include
cellulose resins, such as, ethylcellulose, hydroxyethylcellulose,
ethylhydroxyethylcellulose, hydroxypropylcellulose, cellulose acetate, and
cellulose acetate butyrate; vinyl resins, such as, polyvinyl alcohol,
polyvinyl pyrrolidone, polyvinyl acetate, vinyl alcohol/vinyl butyral
copolymers; polyacrylamide resins, and acrylic acid resins, such as,
poly(methyl methacrylate).
Desirably the weight ratio of dye or other image-forming material to binder
is in the range of from about 0.3:1 to about 2.55:1, preferably about
0.55:1 to about 1.5:1.
The polymer system of the image-receiving sheet serves to enhance the
receipt of dye or other image-forming material in the receiving sheet.
Suitable polymer(s) which can be used as the image-receiving material must
be able to receive dye (or other image-forming material) in order to
maximize dye transfer. The polymer(s) used as the image-receiving material
can also serve to provide mechanical strength to the receiving sheet and
the finished image produced therefrom. Examples of such materials are
extruded polymer films wherein the particular polymer chosen is both
capable of receiving the image-forming material and providing the
necessary mechanical strength.
Polymers which can be used as the image-receiving material include any of
those commonly employed in the art as receiving materials provided they
are immiscible with the polymer system of the donor sheet. For example, a
polyester, polyacrylate, polycarbonate, poly(4-vinylpyridine), polyvinyl
acetate, polystyrene and its copolymers, polyurethane, polyamide,
polyvinyl chloride, polyacrylonitrile or a polymeric liquid crystal resin
may be used as the image-receiving component. Desirably, the polymer for
the image-receiving sheet is a polyester resin, preferably a polyester
resin comprising aromatic diacids and aliphatic diols e.g., Vylon.RTM.
103, Vylon.RTM. 200, and Vylon.RTM. MD-1200 (an aqueous polyester), all
commercially available from Toyobo Co., Ltd., Tokyo, Japan and Vitel.RTM.
2200 and Vitel.RTM. 2700 commercially available from Goodyear Tire and
Rubber Co., Polyester Division, Apple Grove, W.V. Silicone-based oils,
poly(organosiloxanes), fluorine-based polymers, fluorine- or
phosphate-containing surfactants, fatty acid surfactants and waxes are not
suitable compounds to be used as image-receiving materials since they are
not very good at receiving and holding onto dyes.
The thickness of the image-receiving layer will generally be in the range
of about 0.5 to 5 microns (.mu.).
As noted above, the donor binder and receiving polymer(s) must be chosen
such that they are immiscible with each other, upon contact and softening
at the temperature and time of processing, so that no thermal adhesion of
the two sheets will occur during processing. A single polymer as binder
for the donor and a single polymer as the image-receiving material for the
receiving sheet would be preferable; however, it may be necessary to use
polymer blends in the donor and/or receiving sheet in order to optimize
performance for a given system. The polymer blend chosen for either the
donor or receiving sheet may be a homogeneous or heterogeneous blend.
In determining whether two polymers are immiscible one can look to the
relevant art, wherein many studies of polymer-polymer
compatibility/miscibility have been reported, to find pairs of polymers
reported as immiscible. Alternatively, one may employ one of the several
techniques which exist in the art to measure polymer-polymer miscibility.
For a review of these various techniques see The Encyclopedia of Polymer
Science and Engineering, John Wiley & Sons, 1985, vol. 3, pp. 760-765.
However, these techniques result in measures of miscibility which are
relative rather than absolute and depend upon the method of preparation of
the polymer blend. Thus, where a polymer blend is found to be immiscible
using one technique, another may indicate miscibility. For example, the
degree of transparency of the polymer blend is employed as a measure of
immiscibility. If the blend is transparent, it generally indicates the
polymers are miscible; if translucent or opaque, it generally implies
multiple phases and therefore, immiscibility. However, if the refractive
indices of the two polymers are close or equal to each other or if the
domains in a multiphase blend are smaller than the wavelength of light,
the polymer blend may appear transparent even if the two polymers are
immiscible.
In addition, miscibility between two polymers is affected by the presence
of other substances and, therefore, the dye or other image-forming
material in the donor sheet affects the interactions of the donor binder
with the receiving-polymer and can influence miscibility. Additionally,
the method of coating or choice of solvent from which to coat the polymer
blend can impact miscibility. Thus, while determining immiscibility of the
donor binder and receiving polymer by one of the available techniques or
by locating a pair of polymers found to be immiscible in the literature
does not insure that they will work for purposes of the present invention,
it is a good starting point. Routine testing under the conditions of the
present invention will readily determine if a preliminary finding of
immiscibility is maintained under processing conditions.
When a support is employed in the image-receiving sheet, it serves to
provide mechanical strength to the receiving sheet and the finished image.
The support is not particularly limited, although preferably it should
have a thickness of at least 100 microns (.mu.) and desirably 125 to 225
.mu.. If the support is of a thickness less than 100 .mu., it is
susceptible to thermal deformation during printing. The support may be a
sheet or film and may be transparent or reflective. Examples of
transparent supports include polyesters, polycarbonates, polystyrenes,
cellulose esters, polyolefins, polysulfones, polyimides and polyethylene
terephthalate. Reflective supports useful for the image-receiving sheet
include cellulose paper, polyester coated cellulose paper, polymer coated
cellulose paper, e.g., polyethylene or polypropylene coated paper, coated
or uncoated wood-free paper, synthetic paper, and plastic films which
carry a layer of reflective pigment or which include a filler, e.g.,
polyethylene terephthalate containing calcium carbonate or titanium
dioxide. Also useful is a polyester film made opaque by the presence of
voids, commercially available under the tradename "Melinex" from Imperial
Chemical Industries (ICI) Films, England.
To avoid peeling or other damage to the image-receiving layer and/or the
finished image due to poor adhesion of the image-receiving material to the
support, a subcoat may be added to the face of the support which carries
the image-receiving material to enhance adhesion. For example, an anionic
aliphatic polyester urethane polymer, applied as a subcoat, has been found
to enhance adhesion to polyethylene cladded support materials.
The donor sheets used in the present invention can be those conventionally
used in thermal dye diffusion transfer imaging systems. In systems of this
type the image-forming material in the donor sheet is a dye. The dyes that
can be used in the present process can be any of those used in prior art
thermal diffusion or sublimation transfer processes. Typically, such a dye
is a heat-sublimable dye having a molecular weight of the order of about
150 to 800, preferably 350 to 700. In choosing a specific dye for a
particular application, it may be necessary to take account of factors
such as heat sublimation temperature, chromaticity, compatibility with any
binder used in the donor sheet and compatibility with any image receiving
materials on the receiving sheet. Specific dyes previously found to be
useful include:
Color Index (C.I.) Yellows Nos. 3, 7, 23, 51, 54, 60 and 79;
C.I. Disperse Blues Nos. 14, 19, 24, 26, 56, 72, 87, 154, 165, 287, 301,
and 334;
C.I. Disperse Reds Nos. 1, 59, 60, 73, 135, 146 and 167;
C.I. Disperse Violets Nos. 4, 13, 31, 36 and 56;
C.I. Solvent Violet No. 13;
C.I. Solvent Black No. 3;
C.I. Solvent Green No. 3;
C.I. Solvent Yellows Nos. 14, 16, 29 and 56;
C.I. Solvent Blues Nos. 11, 35, 36, 49, 50 63, 97, 70, 105 and 111; and
C.I. Solvent Reds Nos. 18, 19, 23, 24, 25, 81, 135, 143, 146 and 182.
One specific set of dyes which have been found to give good results in a
three-color thermal imaging process of the present invention are:
Yellow C.I. Disperse Yellow No. 231, also known as Foron Brilliant Yellow
S-6GL;
Cyan C.I. Solvent Blue No. 63, C.I. No. 61520, 1-(3,
-methylphenyl)amino-4-methylaminoanthraquinone;
Magenta A [mixture of approximately equal amounts of C.I. Disperse Red No.
60, C.I. No. 60756, 1-amino-2-phenoxy-4-hydroxyanthraquinone, and C.I.
Disperse Violet No. 26, C.I. No. 62025,
1,4-diamino-2,3-diphenoxyanthraquinone].
The donor sheets of the present invention may also be those used in thermal
transfer systems which utilize in situ dye generation to form images. In
systems of this type, the image-forming material in the donor sheet is a
material which, upon application of heat, transfers to the receiving
sheet. The transferred image-forming component combines with a material
already present in the receiving sheet to generate the desired color. Such
systems are described, e.g., in U.S. Pat. No. 4,824,822 and U.S. Pat. No.
5,011,811.
The donor sheet used in the present process conveniently comprises a layer
of image-forming material disposed on one face of the support, the layer
comprising the image-forming material and a binder for the image-forming
material. During thermal imaging, the layer of image-forming material on
the support faces the receiving sheet. The support may be paper, for
example condenser paper, or a plastic film, for example an aromatic
polyamide film, a polyester film, a polystyrene film, a polysulfone film,
a polyimide film or a polyvinyl film. The thickness of the support is
usually in the range of about 2 .mu. to about 10 .mu., although it is
desirable to keep the thickness of the support in the range of about 4 to
about 7 .mu., since a thick support delays heat transfer from the printing
head to the dye and may affect the resolution of the image produced. A
donor sheet having a 6 .mu. polyethylene terephthalate support has been
found to give good results in the present process.
Desirably, a layer of a lubricating agent is present on the back of the
donor sheet remote from the dye layer, the lubricating agent serving to
reduce adhesion of a thermal printing head to the donor sheet. Such a
layer of lubricating agent (also called "heat-resistant slipping layers"),
and methods for its creation on a donor sheet are described in detail in
U.S. Pat. No. 4,720,480, issued Jan. 19, 1988, and hence such lubricating
agents will not be described in detail herein. A preferred lubricating
agent comprises (a) a reaction product between polyvinyl butyral and an
isocyanate; (b) an alkali metal salt or an alkaline earth metal salt of a
phosphoric acid ester; and (c) a filler. This lubricating agent may also
comprise a phosphoric acid ester free of salts.
The filler used in this preferred lubricating agent can be an inorganic or
organic filler having heat resistance, for example, clay, talc, a zeolite,
an aluminosilicate, calcium carbonate, polytetrafluoroethylene powder,
zinc oxide, titanium oxide, magnesium oxide, silica and carbon. Good
results have been achieved in the present process using a lubricating
layer containing as filler talc particles with an average size of 1 to 5
.mu..
Because it is desirable to keep the donor sheet thin, for reasons already
discussed above, the thickness of the lubricating layer preferably does
not exceed about 5 .mu..
The heat required for thermal transfer may be provided by a thermal
printhead or by any other suitable means, e.g., by irradiation with a
laser beam as known in the art.
The present invention is described in more detail by the following
examples.
The sheet materials of each example were thermally processed using a
Hitachi VY-200 thermal printer, sold by Hitachi Ltd., Tokyo, Japan, to
print a multi-color test pattern.
All optical reflection densities were measured using an X-Rite 338
photographic densitometer.
EXAMPLE 1
This Example illustrates the preparation of a sheet material according to
the present invention and its use in thermal imaging. The donor sheet
comprised a support layer of polyethylene terephthalate carrying a dye
layer comprised of dye dispersed in poly(methyl methacrylate) (PMMA). The
donor sheet was in the form of a long roll comprising a plurality of
panes, each pane containing a single color dye or dye mixture, with
yellow, cyan and magenta panes being repeated cyclically along the film so
that each triplet of three panes contained one pane of each color. One
triplet of three panes is used for each print. The yellow pane comprised
two pyridone dyes. The cyan pane comprised two anthraquinone dyes, and the
magenta pane comprised three anthraquinone dyes.
The literature, e.g., Journal of Applied Polymer Science, 41 (11-12) pp.
2691-2704 (1990), has reported that poly(caprolactone) (PCL) is
incompatible with PMMA, and therefore, a receiving sheet was prepared with
PCL as the dye receiving material. A 10% w/v solution of PCL in chloroform
was coated with a Meyer rod (#20) onto a 4 rail (100 .mu. thick)
6".times.6" (15.times.15 cm) opaque polyester terephthalate support
containing voids containing titanium dioxide (commercially available under
the trade name Melinex.RTM. 329, from Imperial Chemical Industries (ICI)
Films, England), and dried in a ventilation hood at room temperature. The
thickness of PCL was approximately 2.mu.. The coated sheet was cut to
size, and the sheet was thermally printed. There was no sticking of the
donor and receiving sheets. The measured dye densities are reported in
Table I.
TABLE 1
______________________________________
DYE DENSITIES
Black Cyan Magenta Yellow
______________________________________
Example 1 1.00 0.95 0.78 0.43
______________________________________
The foregoing data demonstrates that PCL and PMMA maintain their
immiscibility under the thermal processing conditions of Example 1 and
thus prevent sticking of the donor and receiving sheet during thermal
processing. The data in Table 1 show that PCL receives dye.
EXAMPLE 2
A receiving sheet was prepared and processed as in Example 1, except that
the polyester resin, Vylon.RTM. 200, replaced the PCL. This system
exhibited essentially total sticking of the donor and receiving sheets
during thermal processing indicating the combination of PMMA and
Vylon.RTM. 200 for the donor and receiving sheet materials were not
immiscible.
EXAMPLE 3
PCL was blended with Vylon.RTM. 200, the polyester resin of Example 2.
Five-sheet materials were prepared and processed according to Example 1
except that the image-receiving sheets were prepared as follows: varying
ratios of a solution of 16.8% (w/v) Vylon.RTM. 200 in methyl ethyl ketone
(MEK) and a 10% (w/v) solution of PCL in chloroform were mixed and coated
onto a 4 mil Melinex.RTM. 329 support with a #20 Meyer Rod and dried at
room temperature in a ventilation hood to yield a thickness of
approximately 2 .mu.. The percentage (w/w) of PCL in each receiving sheet
is reported in Table 2 as are the measured reflectance densities for the
cyan, magenta and yellow regions and the visible reflection density for
the black region of the test pattern. With 9.3% (w/w) PCL in the receiving
material, there was significant sticking and consequently the dye
densities could not be measured; however, at all other percentages of PCL
reported in Table 2, no sticking was observed. To provide a control, the
experiment was repeated using an experimental receiving sheet comprising a
Melinex.RTM. 329 support and a dye receiving layer comprising a polyester
resin for receiving the dye and a thermally cured silicone release
material comprising an epoxy-modified silicone oil and amino-modified
silicone oil. The reflectance densities-for the control are shown in Table
2. There was no sticking observed for the control.
From the data it can be seen that at 9.3 (w/w) % PCL, there is significant
sticking indicating that under those particular conditions, immiscibility
between the donor sheet and receiving polymer system is not maintained.
However, at higher concentrations of PCL, e.g., sticking was avoided.
Further, at PCL concentrations of about 11%, processing led to
significantly higher dye densities as compared with the control which
utilized a crosslinked silicone release material to prevent sticking. The
data also demonstrate how polymer blends can be utilized in the receiving
sheet to improve performance for a given system, i.e., absence of sticking
and high transferred dye densities.
TABLE 2
______________________________________
DYE DENSITIES
Black Cyan Magenta Yellow
______________________________________
9.3% PCL
Significant
Significant
Significant
Significant
Sticking Sticking Sticking
Sticking
11.1% PCL
2.51 2.05 2.51 2.12
11.5% PCL
2.41 1.95 2.37 2.04
12.4% PCL
2.08 1.68 2.15 1.60
14.4% PCL
2.12 1.70 2.21 1.58
Control 2.36 1.69 2.10 1.53
______________________________________
EXAMPLE 4
This Example illustrates two additional sheet materials according to the
present invention.
Based on their structural similarity to poly(caprolactone), two additional
aliphatic polyesters, poly(2,2-dimethyl-l,3-propylene succinate) (PDPS)
and poly(ethylene adipate) (PEA), were tested for their immiscibility with
PMMA, the binder for the donor sheet, in a sheet material according to the
present invention.
Two receiving sheets were prepared as in Example 3, except that the
receiving material for one was a mixture of PDPS and Vylon.RTM. 200
containing 9.6 wt. % PDPS, and the receiving material for the other
employed a mixture of PEA and Vylon.RTM. 200 (16.3 w/w % PEA). The donor
sheet was the donor sheet described in Example 1, which uses PMMA as the
binder for the dyes. There was no sticking of the donor and receiving
sheets with either receiving sheet upon thermal processing. The measured
reflectance densities are reported in Table 3.
From the foregoing data, it will be seen that the sheet material prepared
according to the present invention did not result in sticking of the donor
and receiving sheets during processing and produced images having good
reflectance densities.
TABLE 3
______________________________________
DYE DENSITIES
Black Cyan Magenta Yellow
______________________________________
9.6 wt. % 2.44 2.10 2.56 2.25
PDPS/Vylon .RTM. 200
16.3 wt. % 2.44 2.02 2.50 2.26
PEA/Vylon .RTM. 200
______________________________________
EXAMPLE 5
This Example illustrates the preparation of sheet materials according to
the present invention and the use of these sheet materials in thermal
imaging. This Example also repeats the experiments using a control which
contains a crosslinked silicone release material to prevent sticking.
Two different receiving materials according to the present invention were
prepared and coated onto various support materials to yield coated
coverages approximately 2 .mu. in thickness in accordance with Example 1.
The two receiving materials were 1) a 10% (w/v) mixture of Vylon.RTM.
200/PEA, (83.6/16.4 w/w %) in MEK and 2) a mixture of Vylon.RTM. 200/PCL
(83/17, w/w %) in MEK:methylene chloride (CH.sub.2 Cl.sub.2), prepared by
combining 7.7 g of a 10% (w/v) solution of PCL/CH.sub.2 Cl.sub.2 with 37.7
g of a 10% (w/v) solution of Vylon.RTM. 200/MEK. These receiving materials
were each coated (using a #20 Meyer rod) onto separate 4 mil Melinex.RTM.
329 supports, 2 mil Toyobo K 1553 synthetic paper (made of polyethylene
terephthalate compounded with fillers) available from Toyobo Co., Ltd.,
Tokyo, Japan, and in the case of Vylon.RTM. 200/PEA on an experimental
paper comprising pigmented polyethylene terephthalate on a cellulose core.
The coated receiving sheets were dried at room temperature. These
image-receiving sheets were used in conjunction with the donor sheet of
Example 1 and processed. There was no sticking of the donor and receiving
sheets for any of the sheet materials during thermal processing. The
reflectance densities are shown in Table 4. To provide a control, the
experiment was repeated with a different receiving sheet. The receiving
material for the control contained a mixture of Vylon.RTM. 200 and a
release material comprising 2.5 w/w % of epoxy modified/amino modified
silicone oils. This mixture was combined with a 50/50 v/v solution of
MEK/toluene to yield a 10% solids solution and was coated with a #20 Meyer
rod to yield a thickness of approximately 2 .mu. onto the above 3
supports, Melinex.RTM. 329, Toyobo and the experimental paper. The
resulting sheets were heated for 5 minutes at 110.degree. C. to cure the
release material. The receiving sheet employing the Toyobo K 1553 support
warped during the thermal curing, but it could still be processed;
however, the experimental paper support became so distorted during the
curing, it could not be put through the printer. The measured reflection
densities for the controls are also shown in Table 4.
TABLE 4
______________________________________
DYE DENSITIES
Black Cyan Magenta Yellow
______________________________________
Vylon .RTM./PEA:
(Melinex .RTM.)
2.29 1.70 2.32 2.09
(Toyobo) 2.06 1.60 2.21 1.85
(Experimental
2.64 1.77 2.73 2.46
Paper Support)
Vylon .RTM./PCL:
(Melinex .RTM.)
2.43 1.69 2.50 2.21
(Toyobo) 2.10 1.60 2.33 1.98
Control:
(Melinex .RTM.)
2.06 1.57 2.26 1.55
(Toyobo) 1.88 1.54 2.17 1.64
(Experimental
-- -- -- --
Paper Support*)
______________________________________
*Could not be thermally printed due to warping.
From the foregoing data it can be seen that the process of the present
invention produced images having significantly increased reflection
density as compared with the control. The experimental data of Example 5
also demonstrate that the support materials which can be used according to
the present invention are not as limited as those which can be used where
thermal crosslinking of a release material is employed to prevent
sticking. The sheet material of the present invention can be dried at low
temperatures, room temperature when organic solvents are used, thereby
avoiding the warping which can occur to heat-sensitive supports during
thermal curing.
EXAMPLE 6
This example illustrates the preparation of a sheet material according to
the present invention and its use in thermal imaging.
The donor sheet is a commercially available material sold by Hitachi, Ltd.,
Tokyo, Japan designated Hitachi Cassette Color Video Printer Paper Ink
Set, VY-SX100 A, high density 100 Series.
The donor sheet is believed to comprise a support layer of polyethylene
terephthalate 10 .mu. in thickness. The support layer carries a dye layer
which is 4 .mu. to 5 .mu. in thickness and comprises dye dispersed in a
vinyl alcohol/vinyl butyral copolymer, which softens at 85.degree. C. and
serves as a binder for the dye.
The donor sheet is supplied commercially in a cartridge comprising a feed
or supply spool and a take-up spool, the two spools having parallel axes
and each being disposed within a substantially light-proof, cylindrical,
synthetic resin housing. The opposed ends of the two cylindrical housings
are interconnected by a pair of parallel rails, leaving between the two
housings an open rectangular frame in which a single pane of the donor
sheet can be exposed.
In the commercial cartridge, the donor sheet is in the form of a long roll
comprising a plurality of panes, each pane containing a single color dye,
with yellow, cyan and magenta panes being repeated cyclically along the
film so that each triplet of three panes contains one pane of each color.
One triplet of three panes is used for each print. The dyes used are
believed to be as follows:
Yellow C.I. Disperse Yellow No. 231, also known as Foron Brilliant Yellow
S-6GL;
Cyan C.I. Solvent Blue No. 63, C.I. No. 61520,
1-(3'-methylphenyl)amino-4-methylaminoanthraquinone;
Magenta A [mixture of approximately equal amounts of C.I. Disperse Red No.
60, C.I. No. 60756, 1-amino-2-phenoxy-4-hydroxyanthraquinone, and C.I.
Disperse Violet No. 26, C.I. No. 62025,
1,4-diamino--2,3-diphenoxyanthraquinone].
The literature, e.g., A. Dondos and E. Pierri, Polymer Bulletin (Berlin)
16(6), pp. 567-569 (1986), has reported the incompatibility of polyvinyl
acetate and polystyrene (PS). Based on the similarity in structure between
polyvinyl acetate and vinyl alcohol/vinyl butyral copolymer, i.e., both
are aliphatic polymers containing polar groups, PS was used as the
image-receiving polymer for the receiving sheet.
Thus, a receiving sheet was prepared according to Example 1, except that PS
replaced the PCL. The donor and receiving sheet were processed according
to Example 1. There was no sticking of the donor and receiving sheets
during processing. The measured reflectance densities are reported in
Table 5.
TABLE 5
______________________________________
DYE DENSITIES
Black Cyan Magenta Yellow
______________________________________
Example 6 0.87 1.14 1.03 0.45
______________________________________
The foregoing data show that the vinyl alcohol/vinyl butyral copolymer and
polystyrene maintain their incompatibility under the conditions of the
present Example and that polystyrene receives dye.
It should be noted that Vylon.RTM. 200 used in Example 2 results in severe
sticking when used by itself as the receiving material with the donor of
this example.
Example 7
Liquid crystal polymers (LCP) have been disclosed as useful materials for
receiving dyes and result in good dye densities, see U.S. Pat. No.
5,024,989, issued Jun. 18, 1991 to the same assignee as the present
invention. However, LCPs have been found to cause undesirable sticking
when used in conjunction with the donor sheet of Example 6. To prevent
sticking and also achieve good dye densities, a receiving sheet was
prepared using a blend of polystyrene and a LCP of the formula
##STR1##
prepared according to the procedure described in the aforementioned U.S.
Pat. No. 5,024,989. A 5% w/v solution of LCP in chloroform was combined
with a 5% solution of PS in MEK to give a mixture containing 7.75% (w/w)
PS/LCP. The resulting mixture was coated with a #20 Meyer Rod to yield a
thickness of receiving material .about.2 .mu. after drying. This receiving
sheet and the donor sheet as described in Example 6 were thermally imaged.
No sticking occurred during processing. The measured reflectance densities
are reported in Table 6. To provide a control, the experiment was repeated
using the commercial donor sheet described in Example 6 and a commercial
receiving sheet, also sold by Hitachi, Ltd., as part of the set for use
with the commercial donor. The receiving sheet is separately designated
Hitachi Video Print Paper VY-S.
The commercial receiving sheet is believed to comprise a support layer
formed of polyethylene terephthalate film 150 .mu. in thickness and
containing pigment particles, which act as an opacifying agent and render
the base layer white in color, so that the images produced on the
receiving sheet are seen against a white background. One face of the
support layer carries a subcoat which is 8 to 10 .mu. in thickness and,
superimposed over this subcoat, an image receiving layer, which is 1.5 to
2 .mu. in thickness and composed of a polyester resin. Additionally it is
believed that the receiving sheet contains a release agent comprised of a
crosslinked siloxane material. The subcoat serves to increase the adhesion
of the image receiving layer to the underlying support layer. There was no
sticking of the donor and receiving sheets during processing. The measured
reflectance densities are shown in Table 6.
TABLE 6
______________________________________
DYE DENSITIES
Black Cyan Magenta Yellow
______________________________________
Example 7 1.92 1.83 2.08 1.37
Control 1.72 1.70 1.96 1.20
______________________________________
The foregoing data, particularly the data in Table 6, show that the process
of the present invention produced images having significantly increased
reflectance density relative to the control.
Since certain changes may be made in the herein described subject matter
without departing from the scope of the invention herein involved, it is
intended that all matter contained in the above description and Examples
be interpreted as illustrative and not in a limiting sense.
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